The majority of such infections are caused by extraintestinal pathogenic Escherichia coli – dubbed ExPEC, for short -- but physicians and researchers have long assumed the bacteria involved always originated in the patient's own body, according to Amee Manges, PhD, of McGill University in Montreal, and colleagues.

But ExPEC outbreaks in several countries in recent years hint at an external source, the researchers reported in the March issue of Emerging Infectious Diseases.

The source is probably retail meat, they argue, and the reservoir is most probably chicken.

That conclusion is based on genetic analysis comparing E. coli from samples of beef, pork, and chicken with strains obtained from humans seeking treatment for a urinary tract infection.

"We suspect that the transmission is occurring the same way other foodborne agents are transferred," Manges told MedPage Today in an email – such things as poor food handling or preparation, kitchen cross-contamination, and undercooking.

But, the researchers argued, there's also a public health implication: Prevention of some human infection might need to start with intervention on chicken farms.

Each year, between six and eight million urinary tract infections are diagnosed in the U.S. with direct healthcare costs for uncomplicated cases reaching as much as $2 billion.

The rise in antibiotic-resistant strains is making treatment more complicated and increasing costs, they added, so that understanding the pathogenesis of the illness is becoming increasingly important.

Manges and colleagues have recently shown that E coli strains that caused community-acquired cases of urinary tract infection can also be found in retail chicken meat.

But it was not clear if other forms of meat played a role or if the transmission was human-to-human (through contamination of food) or if the source was the animal itself.

If the latter is the case, the assumption would be that consumption of the contaminated meat leads to acquisition of the organism in the intestine, which then causes the urinary tract infection.

To clarify the issue, they tested E coli isolates from beef, pork, and chicken, both in shops and in slaughterhouses. They also tested isolates taken from humans with urinary tract infections.

In the case of retail meats, genetic analysis found 15 clonal groups including 63 isolates, 41 of them from retail meat and the rest from humans, they reported.

But although two-thirds of the overall number of retail meat samples were from beef and pork, they made up only 29% of the isolates in the clonal groups, while isolates from chicken accounted for the remaining 71% -- significantly greater than expected at P<0.001.

That implies that isolates from retail beef and pork are much less likely than chicken to be clonally related to isolates from people with urinary tract infections, the researchers concluded.

A similar analysis, looking at cecal samples from animals in abattoirs, found eight clonal groups, including 29 isolates from animals and 17 from humans. Again, the proportion of the isolates from chicken was significantly higher than expected (at P<0.001) – 79% versus 60%.

That finding suggests that the animals themselves can be the source of extraintestinal pathogenic E coli, rather than an intermediary between humans, the researchers argued.

Taken together, Manges and colleagues concluded, "the results suggest that potential ExPEC transmission from food animal sources is likely to be implicated in human infections and that chicken is a major reservoir."

The study had support from the Public Health Agency of Canada, the Canadian Institutes of Health Research, and McGill University. The authors did not report any potential conflicts.

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